• An artillery shell has a mass of 55 kg. The projectile is fired from the piece and has a velocity of 770 m/s when it leaves the barrel. The gun barrel is 1.5 m long. Assuming the force and therefore the acceleration is constant while the projectile is in the barrel, what is the force that acted on the projectile?

Solution

• Find a: 2

2 2 22 0 22ov

v v ax v ax ax

25

2 2

1770 197 600 2.0 10

2 1.5

m m ma x

s m s s

Find the force:

5 5 72

55 2.0 10 110 10 1.1 10m

F ma kg x x N x Ns

The Third Law:

Third Law If two objects interact, the force exerted on object 1 by object 2 is

equal in magnitude but opposite in direction to the force exerted on object 2

by object 1.

• The classic way of saying this is, “For every action there is an equal and opposite reaction”.

• Newton’s third law simply says that forces come in pairs. You push on a wall and the wall pushes on you. We call these action/reaction force pairs.

• One of the skills most people master is walking. We rarely think about the act of walking – you don’t have to concentrate on it, it’s just something that you can do. It turns out, however, that walking is a fairly sophisticated application of Newton’s laws of motion.

• The key to walking is the third law. You push against the ground and the ground pushes you. All there is to it. But how does that make you move? Okay, you takes your basic second law. It goes like this; you exert a force on the ground and it exerts a force on you. The two forces are equal and opposite. The force exerted on you by the earth causes you to accelerate. The size of the acceleration depends on the force and your mass. F = ma. Because your mass isn’t very big compared to the earth, you end up with a pretty good acceleration – enough to make you move.

• What about the earth? It also had a force exerted on it – the one from your foot pushing on it. Does it also get accelerated? Well, yes, it has to. The same magnitude force is acting on it. Why then don’t you notice the earth moving away from you? The second law is involved here. F = ma, but this time the mass of the earth is huge (6.0 x 1024 kg ) compared to your mass. If your mass is 60 kg, the earth is 1023 times more massive than you. That means that the acceleration acting on the earth is 1023 times smaller than your acceleration. It is so small that it cannot be measured.

But it is real.Because the forces are the same, the mass times the acceleration has to equal the same thing. So

we get:

mear t h you am a

ear t h=

you

Rockets work because of the third law. A common misconception is that a rocket works by pushing against air. We know this is not true because rockets work in outer space where there is no air to push against. The way a rocket works is like this: hot gases, the products of combustion, are blasted out of the end of the rocket. They are pushed away from the rocket, but according to the third law if the rocket pushes the exhaust gases away, the exhaust gases must push the rocket in the opposite direction. So the rocket goes.

Rocket pushesgas

gas pushesr ocket

The Third Law:

• Here is the classic third law conundrum. An acquaintance tells you, “There’s no way a horse can pull a wagon.”

• You say, “No way that’s true because horses pull wagons all the time!” (You’ve been around, you see, and no one can fool with you.)

• “No, I’m right. The horse can’t pull a wagon. See, if the horse pulls on a wagon, then according to the third law the wagon has to pull on the horse with an equal and opposite force. Since the two forces are equal and opposite, they cancel out, so the horse can’t pull the cart!”

The argument sort of makes sense, but you know that it has to be flawed because it is clearly not true. So what’s

the deal?

Action/reaction force pairs never cancel each other out – they can’t because they are partners in the same action force. Can you touch your mother without being touched by her? Does your mother touching your finger when your finger touches her cancel out the touch? Can a single-hand clap?

The Third Law:• In order to have forces cancel out; they have to be different forces.

An example of this would be if we had two horses hitched to the same wagon pulling in opposite directions. The forces the horses exert would then cancel out and the wagon wouldn’t move. These would be two separate forces that do cancel out.

• The horse does indeed pull on the wagon and the wagon pulls on the horse in accordance with the third law. The wagon pulling on the horse results in keeping the horse from running off really fast – the horse is slowed down by the extra mass of the cart, so its acceleration is much smaller than if it were not hitched to the wagon.

• The horse moves, dragging the wagon with it, by pushing the earth away from it. The earth pushes back on the horse, so the horse and wagon move. The earth moves away as well, but, because its mass is so huge (as previously discussed), we can’t measure its acceleration.

What does this picture show about Newton’s laws?

Newton's Second Thought Laws:•1) A body at rest tends to watch television.•2) A body acted upon by a force usually breaks.•3) For every human action there is an overreaction.

HW pg 125-129

• Take notes

• Answer 1-5 an page 129